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Is it safe to charge your phone overnight?

Ever since the smartphone became an integral part of our daily lives, many different, often contradictory statements have been made about the behaviour of their batteries. In particular, the answer to this existential question has been the subject of intense debate in many hoseholds. Should you charge your phone overnight?

Today, we finally have a clear answer, and it is actually not that complex.

Current batteries technology explained

All modern smartphones use lithium-ion (Li-ion) batteries. This technology, existing since 1991 (first commercialized by Sony), has massively replaced the older nickel-based batteries, which, due to their different nature and composition, used to require a different type of care. Even though these have now largely dissapeared, they have left behind many false myths and incorrect behaviours.

For instance, we have all heard before this widespread piece of advice:

Before your first charge, make sure you let the battery drop to 0% and then charge it completely to 100%”

This is a great example of a common misconception regarding current batteries’ operation. While it used to be true for some of the older nickel-based batteries, this is no longer the case for cuzrrent smartphone’s batteries. In fact, it can actually do more harm than good, as we will see soon.

Is it actually safe to charge a phone overnight?

Let’s start answering this by stating a fact: all smartphones have an inbuilt charge controller chip which prevents it from being overcharged. This means that, as soon as the battery level reaches 100%, your phone essentially disconnects the battery from the plug, protecting the battery from overcharging. After this, the phone continues running off the charger, leaving the battery untouched at 100%, and using the plugged-in charger to provide the small power consumption that the phone still demands.

iphone charging overnight

It would thus seem safe to charge your phone overnight, and this is in fact what many people believe. Unfortunately, the issue is not as simple as that. There are other factors that come into play, and the lithium-ion material being used has a big say in this.

Lithium-ion batteries have a lot of advantages over the traditional nickel-based batteries: they can support bigger capacities, they support faster charging, and they do not suffer from the infamous memory effect (more on this later). However, they have a clear disadvantage: their performance degrades over time, particularly when exposed to certain adverse conditions.

Factors affecting the performance of a battery

Manufacturers typically rate their batteries in cycles. A cycle is defined as a usage of 100% of a battery over time, although not necessarily from 100% to 0%, but rather accumulative (for example, from 100% to 30%, and then from 100% to 70% the next day).

Apple, for instance, states that the iPhone battery retains only 80% of its origina capacity after 500 cycles. This estimate is industry-average for most Li-ion battery manufacturers.

As it turns out, the number of cycles that a battery has gone through is the most important factor affecting battery life. Unsurprisingly, this means that the more a battery gets used, the lower it’s maximum capacity becomes. Simple, right? Well, not really.

There are two other relevant factors which, at an equal number of battery cycles, can drastically change a battery’s performance. These are charge level and temperature.

Numerous studies have shown that exposing your battery to extreme battery levels, both high (above 80%) and low (below 20%), creates an irreversible reduction in the battery’s capacity, proportional to the time it remains in that state. In practice, this means that every minute that your phone’s battery spends outside of the safe range (considered to be between 20% – 80%) is damaging your battery. Ideally, it is considered good practice to avoid charging your phone over 80%, as well as to plug it into the charger before it drops under 20%.

Temperature also plays an impotant role in a battery’s lifecycle. It is now widely accepted that continuous exposure of a Li-ion battery to high temperatures causes faster degradation. The main issue here, however, is not so much your average room temperature, which at around 20°C (68°F) tends to be relatively mild, but rather the high temperatures that a battery reaches when charging. This effect can get particularly exacerbated if the device is being used at the same time.

This table illustrates how damaging the effect these two factors (charge level and temperature) can be when compounded together:

Temperature40% charge100% charge
0°C98% (after 1 year)94% (after 1 year)
20°C96% (after 1 year)80% (after 1 year)
40°C85% (after 1 year)65% (after 1 year)
60°C75% (after 1 year)60% (after 3 months)
Battery capacity (as a % of original capacity) when exposed to different scenarios. Source: Battery University

While all batteries degraded with increasing temperature, those charged to a level of 40% suffered a much smaller loss in capacity than those fully charged.

Making your battery more durable

Luckily, we live in the year 2020, the era of information where science understands the behaviour of current batteries quite well. Simply follow these recommendations to take care of your phone’s battery, minimize the adverse effect of all of the aforementioned negative factors, and make your phone’s battery last longer in time:

  1. Avoid charging your phone over 80% unless strictly necessary. Of course, not everyone can afford the luxury of leaving the house with less than 80% charge, but if you can, this is the factor that will make the biggest difference.
  2. Make sure you plug your phone before it drops below 20%. Particularly if you are at home, don’t delay it further and plug that charger in before the battery level drops even more.
  3. Avoid using the phone while charging. This causes overheating on the battery, which causes long-term degradation on the anode, which leads to a permanent loss of battery capacity.
  4. Use a fast charger. Even if it seems counter-intuitive, the faster you charge your phone, the better this is for your battery. This is because charging = overheating, and charging a battery fast means a shorter exposure to high temperatures.

The verdict: so, is it safe to charge your phone overnight?

As shown, generally it is NOT a good habit to charge your phone completely overnight. Luckily, there are multiple alternatives you can do to circumvent this. These include using a 80%-limited charger/app, investing in a good fast charger, and ensuring your phone never drops below 20%, among other habits.

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